Those natural properties, such as hardness and roundness, can be perceived and encountered in the real world.
On the other hand, it is not immediately clear how to physically see, touch or measure the goodness of a novel or the rightness of an action.
Ethical intuitionists assert that, if we see a good person or a right action, and our faculty of moral intuition is sufficiently developed and unimpaired, we simply intuit that the person is good or that the action is right.
When someone judges something to be good, or some action to be right, then the person is using the faculty of moral intuition.
Perhaps the best ordinary notion that approximates moral intuition would be the idea of a conscience.
Moore also introduced what is called the open-question argument, a position he later rejected.
In his initial argument, Moore concluded that any similar definition of goodness could be criticized in the same way.